1. Field of Invention
The invention relates to a laser beam scanner that scans a photosensitive medium with a laser beam and exposes an image on the photosensitive medium.
2. Description of Related Art
Laser beam scanners have become widely used in recent years. By way of example, FIG. 8 shows a main structure of a conventional laser beam scanner 100. The laser beam scanner 100 comprises a laser source 101, a cylindrical lens 102, a polygon mirror 103, a first fθ lens 104, a second fθ lens 105, a beam detector 106, a focusing lens 107, and a photosensitive drum 110. The laser source 101 has a semiconductor laser element for emitting a laser beam and a collimate lens that are integrated into a single unit.
The laser beam emitted from the laser source 101 is converged by the cylindrical lens 102 and, then, is deflected by the polygon mirror 103. The deflected laser beam is converged by the first fθ lens 104 and the second fθ lens 105 onto the photosensitive drum 110. Herein, the first fθ lens 104 has the ability to converge the laser beam in a main-scanning direction, while the second fθ lens 105 has the ability to converge the laser beam in a sub-scanning direction.
The laser beam reaches the focusing lens 107 before irradiating the photosensitive drum 110. When the laser beam is converged by the focusing lens 107 to the beam detector 106, the beam detector 106 detects the laser beam, and determines a scan start time to start scanning an image area on the photosensitive drum 110. The beam detector 106 is arranged at a certain position to which the laser beam is converged, but out of the image area on the photosensitive drum 110.
Further, there is provided another type of laser beam scanner. This type of laser beam scanner has a structure similar to the above-explained laser beam scanner, but further comprises a mirror for reflecting a laser beam that has passed through the second fθ lens 105. The laser beam reflected by the mirror is converged by the focusing lens 107, and is detected by the beam detector 106.
The conventional laser beam scanners are applied, for example, to laser printers and digital copiers. The desire to miniaturize laser printers and digital copiers has grown in these application areas in recent times. Especially, saving space around the photosensitive drum 110 and the second fθ lens 105 is an important issue, because both the photosensitive drum 110 and the second fθ lens 105 have great widths in the main-scanning direction and occupy a large amount of space.
However, every conventional laser beam scanner has to comprise the beam detector 106 and the focusing lens 107 arranged around the photosensitive drum 110 and the second fθ lens 105, so that the laser beam that has passed through the second fθ lens 105 is converged to the beam detector 106 just before reaching the photosensitive drum 110. Such an arrangement makes it difficult to save space around the photosensitive drum 110 and the second fθ lens 105.
In addition, the conventional laser beam scanner has various kinds of lens (such as a cylindrical lens, an fθ lens, and a converging lens), as described above, for properly bringing the laser beam into focus on the photosensitive drum 110 and on the beam detector 106. The lenses cannot share their individual functions with the other lenses. Thus, the laser beam scanner must have all of the lenses, thereby leading to high manufacturing cost.
The invention has been developed to resolve the above-mentioned and other problems, and especially to provide a laser beam scanner that scans a photosensitive medium with a laser beam and exposes an image on the photosensitive medium.